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March 9, 2009 -- Children born to older fathers don't perform as well on tests of thinking skills during infancy and early childhood, while those born to older mothers have higher scores on the same tests, a study shows.

This latest study follows previous research showing a link between older fathers and health problems such as birth defects, autism, and schizophrenia, suggesting that the "biological clock" isn't just a concern of women.

"The links emerged in the 1990s that the offspring of older fathers had an increased risk of schizophrenia, [and] since then data has accumulated also linking paternal age to autism and more recently bipolar disorder," says study researcher John McGrath, MD, PhD, a professor at the Queensland Brain Institute of the University of Queensland, Brisbane, Australia.

"We wondered if it was a more generic process," he says, and that triggered their current research to look at potential links between a father's age and child development, including IQ.

The study is published this week in PLoS Medicine.

McGrath's team analyzed data from a large study called the U.S. Collaborative Perinatal Project, which recruited pregnant women from 12 sites in the U.S. from 1959 to 1965. The data from this ongoing project has been a "treasure trove" for researchers, McGrath says.

His team looked at more than 33,000 children born between 1959 and 1965 and then looked at their results on cognitive tests administered at ages 8 months, 4 years, and 7 years. The tests evaluated the children's ability to think and reason, measuring such skills as concentration, learning, speaking, reading, arithmetic, memory, and motor skills such as coordination.

Finally, they looked for links with the father's age, the mother's age, and in one analysis also adjusted for socioeconomic factors such as family income and parental education.

The average age of the fathers in the study was 28.4 and ranged from 14 to 66. The mother's average age was 24.8 and ranged from 12 to 48.

In recent years, according to the paper, it has become very common for couples to delay having children until their late 30s.

Intelligence Tests for Children of Older Fathers

The older the father, the more likely the child was to score lower on the tests, except for one measure of motor skills.

When they looked at the mother's age, however, they found that the older the mother, the higher the children scored on the thinking skills tests. (That finding, reported in earlier studies as well, may be due to a more nurturing home environment if the mother is older, but this study suggests children of older fathers don’t reap the same benefit.)

However, when the researchers adjusted for such factors as the parents' socioeconomic status, including income and education, it modified the effect of both parents' ages on the intelligence tests. For instance, the average score on the Stanford Binet Intelligence Scale was nearly 6 points lower for children born to fathers age 50 compared to those born to fathers age 20. But when the socioeconomic factors were taken into account, the difference dropped to 2.2 points.

While the study findings may suggest the best combination of parents is an older woman with a younger man, McGrath says it's too early to make any specific recommendations.

"For the moment, our study suggests that paternal age, like maternal age, also should be 'on the radar screen'" for researchers, he says. As research accumulates, he says, "we can put this knowledge into the public health equation," weighing it along with many other factors before dispensing advice.

What's behind the link between older fathers and lower IQ? "There is a growing body of evidence suggesting that the sperm of older dads develop more mutations, that is, spelling mistakes in the DNA code," McGrath says. His team is researching this idea further in animal studies comparing young mice with older ones.

Still, it's important to put the paternal age in perspective, McGrath says. "The significance of the effect linking paternal age and child cognition is small compared to many other socio-cultural factors -- for example good prenatal nutrition, good nutrition for the offspring, good education, nurturing home life, excellent teaching and school opportunities [and so on]."

Second Opinions

"We have known about the paternal age effect for many years," says Harry Fisch, MD, director of the Male Reproductive Center and director of urologic microsurgery at Columbia University Medical Center of New York Presbyterian Hospital. Yet, he says, "We are just starting to scratch the surface."

Testosterone levels begin to decline slowly at age 30, Fisch says. Ideally, men should father children "sooner rather than later," he says.

"The 20s and early 30s are ideal, but real life intervenes," he says, making that time frame not feasible due to lack of a partner, difficulty getting pregnant, financial restraints, or a host of other factors.

In a perspective on the study, published in the same issue of PLoS Medicine, Mary Cannon, MD, of the Royal College of Surgeons in Dublin, Ireland, says it is important to take socioeconomic factors into account when looking at the effect of a father's age (as well as a mother's).

She points out that taking into account the socioeconomic factors is not a precise science, and that if researchers could truly adjust for every relevant socioeconomic variable, the effect of the father's age on the child's intelligence might be wiped out completely. She, too, calls for more research.